1. Field of the Invention
This invention relates generally to a piezoelectric element such as a piezoelectric resonator or a surface acoustic wave (SAW) device made of a piezoelectric crystal such as quartz crystal or a piezoelectric ceramic material. More particularly, the present invention relates to a piezoelectric element which is surface treated using an excited active species generated in plasma at approximately atmospheric pressure during manufacture. The invention also relates to a method of manufacturing such a piezoelectric element, and a mounting apparatus for connecting a piezoelectric resonator to an external connecting plug or a lead during the manufacturing process thereof.
2. Description of the Related Art
It is conventionally well known in the art to provide a piezoelectric element which operates in accordance with a piezoelectric effect principle based upon a resonance. Such a piezoelectric element may be employed for use in a resonator, an oscillator or a filter. There are a number of types of piezoelectric elements. These include a piezoelectric element formed as a tuning fork operating within a frequency band ranging from about 20 kHz to 1 MHz, a piezoelectric resonator such as an AT-cut resonator operating within a frequency range of from about 4 to 125 MHz, and a SAW device using a surface acoustic wave.
In general, a piezoelectric resonator is formed of a resonator comprising a piezoelectric piece having an electrode formed on the surface thereof and a plug coupled to the resonator. A lead is also provided for electrically connecting the plug to an outside electrical circuit. Finally, a case housing for the resonator is provided. The resonator is sealed within the case in an air-tight manner. Usually, connection between the resonator and the plug is accomplished by soldering an inner lead terminal of the plug onto a land of the resonator. The reflow process is commonly used as the soldering process, and comprises the steps of plating a predetermined amount of solder onto the surface of the inner lead terminal and then melting the solder with a hot blast or with infrared rays to allow the connection between the plug and the land.
A SAW device, which permits stable achievement of a high frequency within a range of from between 100 MHz to the giga Hz band, may be used to construct a SAW resonator for use in an oscillating circuit and a SAW filter for high frequencies. In general, a SAW device comprises a piezoelectric substrate having a fine comb-shaped electrode formed on the surface thereof. The substrate is sealed in a case in an air-tight manner, and the comb-shaped electrode is connected to an electrode terminal of the case by wire bonding. Recently, the inventors of the present invention have developed a SAW device having an improved construction in which the inner lead of a plug is soldered to the land of a piezoelectric substrate. The SAW device is then sealed in a case in an air-tight manner, as in the above-mentioned conventional piezoelectric resonator.
To remove an oxide film which may be spontaneously formed on the substrate when the substrate comes into contact with ambient air, and for improving the wettability of solder, a flux is often used during the soldering process. However, when using such a flux on a piezoelectric element, residual flux or splash may cause corrosion of the electrode on the piezoelectric piece surface and may in turn deteriorate the performance of the piezoelectric piece. Thus, a process for improving wettability which does not use a flux would be beneficial. However, because such conventional fluxless processes cannot remove the oxide film from the surface of the substrate, i.e., the land, the wettability of the solder greatly varies in accordance with varying surface conditions. This in turn may result in defective soldering and a decrease in the production yield of the piezoelectric pieces.
In order to avoid defective soldering which maybe caused by a low wettability of the employed solder, a number of conventional measures may be employed. These include increasing the heating temperature to the range of about 430 to 450.degree. C. or increasing the amount of solder used. However, when a quartz crystal is employed as the piezoelectric piece, excessive heating may cause distortion, resulting in a twin crystal having deteriorated properties such as CI (crystal impedance) value and temperature coefficient. Additionally, this excessive heating may result in soldering erosion or the thin film land, thus leading to a soldering defect.
The utilization of a larger amount of solder increases the manufacturing cost. Soldering of a deposit having a thickness of over 20 to 25 mm on the surface of the inner lead requires a great amount of time and results in a tendency of the lead to bend. Furthermore, if molten solder does not flow sufficiently around the inner lead, a sufficient mechanical connection strength will not be obtained even though the electrode is physically connected to the lead. Thus, any impact may cause peeling of the resonator from the lead. Finally, adhesion of molten solder to the electrode may cause performance problems such as unstable frequency maintenance by the piezoelectric element or an increase in the CI value. Therefore, it would be beneficial to provide an improved piezoelectric element which overcomes these problems of the prior art.
In order to increase productivity, a batch treatment method may be employed in which a plurality of piezoelectric resonators are surface treated at a time. This method is favorable for improving productivity in an actual manufacturing line. However, during this process of improving wettability, the entire surface of the piezoelectric resonator may be treated. Thus, any solder employed would flow to portions of the electrode which are not to be connected to the lead. This may lead to an immediate degradation of performance of the piezoelectric element, or may also lead to a decrease in the performance of the piezoelectric element over time during use. Therefore, it would also be beneficial to provide a method of manufacture which overcomes these problems of the prior art.